Heretofore, flashlights incorporating light emitting diodes (LEDs) as their light sources have typically employed one or more high intensity, five millimeter LEDs. Typically, white LEDs are used and, because of their characteristic forward voltage drop, have been limited to flashlights employing three or more standard alkaline cells. Costlier units have employed two lithium cells to achieve the required voltage. To limit the current to the correct operating value, a resistor has been used, either within a bulb base or external to the LEDs. This requires that the bulb only be used with the number and type of cells for which the resistor value was selected. With such devices, the end user must, therefore, specify the number of cells with which the bulb will operate.
The use of a resistor as a current limiter is not excessively wasteful of power when LEDs requiring only 30 milliamperes are driven. The extension of such a method for use with the recently released one watt LEDs requiring 350 milliamperes means that, as a minimum, 50 percent of the energy within the battery pack is wasted as heat by the current limiting resistor. As the number of cells in the flashlight increases to greater than two cells, the wasted power increases proportionately.
There exist drive circuits for driving LEDs using battery packs. An example of such a drive circuit is a drive circuit designed by Zetex, PLC for its Zetex Type ZXSC310 LED Driver chip. This drive circuit, schematically depicted in FIG. 5, includes a control chip 550, a switching transistor 560, a current sense resistor 590, a series of LEDs 520, 521, 522 and 523, an inductor 570, a Schottky rectifier 575 and a capacitor 580. The current sensing terminal “CS” of the control chip 550 is directly tied to the junction of the switching transistor 560 and the current sense resistor 590. The cathode of the LED series 520–523 is connected to the negative side of the battery pack.
A first end of the inductor 570 is connected to the positive contact and a second end of the inductor 570 is connected to the anode end of the diode. The collector of the switching transistor 560 is connected to a junction between the second end of the inductor 570 and the anode end of the Schottky rectifier 575. The emitter of the switching transistor 560 is connected to a first end of the current sense resistor 590, and a second end of the current sense resistor 590 is connected to the negative side of the battery pack. The anode of the LED series 520–523 is connected to the cathode end of the Schottky rectifier 575. The output terminal OUT of the control chip 550 is connected to the base of the switching transistor 560. A first end of the capacitor is connected to a junction between the cathode end of the Schottky rectifier 575 and the anode of the LED series 520–523, and a second end of the capacitor is connected to a junction between the cathode of the LED series 520–523 and the negative side of the battery pack.
The control chip 550 provides a signal through the output terminal to the switching transistor 560 to break electrical connection across the transistor 560 from the collector to the emitter when voltage at the current sensing terminal exceeds a threshold value, and provides a signal through the output terminal to the switching transistor 560 to provide electrical connection across the transistor 560 from the collector to the emitter after a non-conduction period has elapsed following the breaking of the electrical connection across the switching transistor 560.
When voltage is applied to the circuit, the OUT terminal of the control chip 550 goes to a positive voltage, driving the switching transistor 560 into a conductive state. Current is drawn through the inductor 570 and increases linearly until the voltage across the current sense resistor 590 reaches a threshold point at the CS terminal of the control chip 550. When the threshold voltage is reached, the OUT terminal of the control chip 550 goes low and the switching transistor 560 ceases to conduct. The energy stored by the inductor 570 during the conduction time of the switching transistor 560 is discharged into the load presented by LEDs 520–523 through the Schottky rectifier 575. In order to maintain a constant current, the voltage across the inductor 570 will rise until the LEDs 520–523 conduct and the remaining energy in the inductor 570 will provide a constant current until the stored energy is exhausted.
A significant drawback of the Zetex circuit for the Zetex Type ZXSC310 LED Driver is that if the battery pack voltage is higher than the sum of the forward voltage drop of the driven LED plus the forward voltage drop of the Schottky rectifier 575, the LED will be subjected to overcurrent and possibly damaged. Additional LEDs are placed in a series connection to assure that the LEDs forward voltage drop is always greater than the supply voltage.
The non-conduction period of the switching transistor 560 is fixed. Therefore, the magnitude of the stored energy is determined by the input voltage, the value of the current sense resistor 590, and the threshold voltage of the CS terminal, the value of which is fixed. If the input voltage is increased, the amount of energy stored increases proportionately. Because the current delivered to the LEDs is a direct function of the stored energy, an increase in input voltage can result in excessive current being applied to the LEDs.